Helix Nebula
From Wikinfo
- For the gigantic nebula discovered in 2006 near the center of the Milky Way Galaxy, see Double Helix Nebula
The Helix Nebula (also known as NGC 7293) is a planetary nebula (PN) about 650 light-years away in the constellation Aquarius. It is one of the closest planetary nebulae to Earth[1] and was discovered by Karl Ludwig Harding before 1824. It has a very similar appearance to the Ring Nebula. It is also similar in size, age, and physical characteristics to the Dumbbell Nebula, with the significant difference in appearance being a consequence of the relative proximity and more nearly equatorial viewing angle of the Dumbbell.[2] The Helix has often been referred to as the Eye of God on the Internet, since about 2003. The Helix Nebula was also featured in the 2004 Battlestar Galactica TV series.
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General information
The Helix Nebula is an example of a planetary nebula created at the end of the life of a Sun-like star. The outer gases of the star expelled into space appear from our vantage point as if we are looking down a helix. The remnant central stellar core, destined to become a white dwarf star, glows in light so energetic that it causes the previously expelled gas to fluoresce.
Helix lies about 650 light-years away towards the constellation of Aquarius and spans about 2.5 light-years. Recent pictures of Helix are a composite of newly released images from the ACS instrument on the Hubble Space Telescope and wide-angle images from the Mosaic Camera on the WIYN 0.9 m Telescope at Kitt Peak National Observatory.
It is estimated to be 10,600Template:± years old based upon an expansion rate of 31 km/s.[2]
Structure
The nebula is thought to be shaped like a prolate spheroid with a strong density concentration toward a filled disk of the equatorial plane. Its major axis is inclined about 21° to 37° from our vantage point. It is composed of an inner disc with a 8′ 19″ diameter (0.52 pc) and outer torus with a 12′ 22″ diameter (0.77 pc) and an outer-most ring with a 25′ diameter (1.76pc). The outer-most ring is flattened on one side due to its colliding with the ambient interstellar medium. The PN is estimated to have been expanding for 6,560 years for the inner disc and 12,100 years for the outer ring with expansion rates of 40 km/s and 32 km/s respectively.
Knots
The Helix Nebula was the first PN discovered to contain knots. The main ring contains knots of nebulosity which have now been detected in many near-by PNs. In the Helix, these knots are highly radially symmetric (from the central star) and "cometary", they contain bright cusps (local photoionization fronts) and tails which extend away from the star in the radial direction. At the cusps the knots are optically thick to Lyc photons from the central star.[2][3][4]
See also
External links
- NASA APOD - May 10, 2003 NGC 7293: The Helix Nebula
- SEDS: NGC 7293, the Helix Nebula
- NightSkyInfo.com - NGC 7293, the Helix Nebula
- Snopes.com; Helix Eye of God Urban Legend
Notes
- ^ 13.5 apparent magnitude - 5 * (log10(242 pc distance) - 1) = 6.58 absolute magnitude
References
- ^ Hora, Joseph L.; Latter, William B.; Smith, Howard A.; Marengo, Massimo (2006). "Infrared Observations of the Helix Planetary Nebula". The Astrophysical Journal 652 (1): 426-441.
- ^ a b c
- ^
- ^ O'dell, C. R.; Balick, B.; Hajian, A. R.; Henney, W. J.; Burkert, A. (2003). "Knots in Planetary Nebulae". Winds, Bubbles, and Explosions: a conference to honor John Dyson, Pátzcuaro, Michoacán, México, September 9-13, 2002 (Eds. S. J. Arthur & W. J. Henney) Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias) (http://www.astroscu.unam.mx/~rmaa/) 15: 29-33.
Adapted from the Wikipedia article, "Helix Nebula" http://en.wikipedia.org/wiki/Helix_Nebula, used under the GNU Free Documentation License.
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